I completely and totally feel your frustration! My son is over 4 and is finally (mostly) trained. I thought it would NEVER happen! We started at two-and-a-half; it was a very long road. My advice is with the moms who said to not make it a power struggle. That's where we made a mistake: after he was three and we knew he was soon going to preschool, we got nervous and really started to push. He'd had some issues with constipation and soon he was withholding his stool. He didn't poop for over ten days, and all day long, about every ten minutes, he'd stop what he was doing to hold it in. It became a medical issue and we had to take him to the doctor for a very strong and painful enema. It was horrible!!!
We thought we had done all the right things; sticker chart, rewards, shopping for new underwear, .........I tried the "potty-train your child in one day" book, which I think is a good book; it just didn't work on our son. I even called and talked to the book author several times looking for advice! Nothing worked, nothing.
After that as per doctor advice, we backed off, left him in pull-ups, and every once and a while we'd ask him if he wanted to wear underwear, go on the potty, etc..... and he'd say no. Eventually, with laxitives, we got beyond the stool issues, things had settled down, but preschool was coming up, so we made up a story about the "pull-up fairy" who came an took his pull-ups for a boy who really needed him, and she left him some cool new underwear, etc.... and we just stopped with the pull-ups. It was a mess for a very, very long time, but we stuck with the "pull-ups are only for bedtime" plan and eventually he "got it."
The ideas about punishing him or making him clean up after himself didn't work for us either. Sticker charts with a "big reward" at the end didn't either. He is much more into instant feedback. We used love, hugs, praises, and of course, candy - skittles and starbursts sit on a shelf above our toilet, and sometimes he still asks for one, sometimes he doesn't. We found, with our son, praising the positive and somewhat ignoring the negative worked. Exceptions to that are "tv watching" and other fun stuff that are "privileges" in our house, and if there is a potty accident, no tv or no play do or whatever is the privilege, because "big boys can watch tv" "boys who pee/poop their pants just are not big boys." That seems to work too.
Lately, he's figured out if he has an accident, he just goes to his room and changes himself, so now we're working on the clean-up part, not as a punishment, but as a logical consequence to making a mess. That would NOT have worked when he was three.
My daughter who is not yet two loves the toilet, loves to sit on it, and occasionally pees in it. She wants to be changed immediately after she goes, and with any luck, she'll be trained a lot earlier, but we're not going to push it.
Every kid is different, and I wish you luck. Hands down, this was my worst experience (so far) as a parent, and I guess when I look at all the other trials parents go through, I consider myself pretty lucky.